The neuroprotective enzyme CYP2D6 increases in the brain with age and is lower in Parkinson's disease patients.
Mann A, Miksys SL, Gaedigk A, Kish SJ, Mash DC, Tyndale RF. The neuroprotective enzyme CYP2D6 increases in the brain with age and is lower in Parkinson's disease patients. Neurobiol Aging. 2012 Sep; 33(9):2160-71.